1. Field of the Invention
The present invention relates to an apparatus and method for judging road surface conditions between a road surface and wheels of a vehicle and particularly to an apparatus and method for judging grip conditions between road a surface and tires.
2. Discussion of Related Arts
In recent years, when control parameters are calculated in miscellaneous vehicle control strategies such as traction control, braking force control and torque distribution control, friction coefficient(s) on road surface and grip conditions between wheel and road surface are frequently referred to. In performing such vehicle controls, it is of importance to make a correct judgment of road surface conditions of a road on which a vehicle presently travels in order to enhance the accuracy of the vehicle controls. However, in case where the vehicle travels on the same road surface with different types of tire, the road surface is erroneously judged. Hence, it is necessary to know a type of tires (summer tire, studless tire etc.) which a vehicle is equipped with before making the judgment of road surface conditions or grip conditions.
For example, Japanese Patent Application Laid-open No. Toku-Kai-Hei 09-188114 (JP-A-09-188114) discloses a method of identifying the type of tire. In which, respective wheel speeds of four wheels and longitudinal acceleration of the vehicle are measured and the ratio of the rotation speed of a front wheel to the rotation speed of a rear wheel is calculated based on these measured values and the type of the tires installed on the vehicle is judged by comparing the function of acceleration—front and rear wheel speed ratio with the calculated the ratio between front wheel speed and the rear wheel speed. The function of acceleration—front and rear wheel speed ratio is formed based on data of the ratio between front wheel speed and rear wheel speed for every specified acceleration with respect to the tires whose type is known.